It is well known in the coating industry that coil coating is a cost effective and efficient method of applying coating compositions onto a number of different substrates, particularly metals. For example, coil coating processes are often used for finishing panels employed in the fabrication of many major appliances such as refrigerators, vending machines, washers, dryers, and other metal cabinetry.
Compositions which are used to coat a metal substrate prior to the metal being formed into its desired shape must be curable to form a coating which is hard, yet flexible, in order for the pre-coated metal to withstand the various post-coating forming operations. In addition, it is often very desirable that, once cured, these types of coating compositions possess a high level of stain resistance and a high gloss (i.e., a 60.degree. gloss value of at least 70%, preferably at least 80%, as determined by ASTM D 523-78). These features impart on the resulting coated substrate not only functionality, but also an attractive appearance to the finished article made therefrom.
In view of the many advantages associated with the implementation of pre-coated metals, those in the coatings industry are continually attempting to formulate improved compositions which can be used for these purposes. One such attempt is set out in U.S. Pat. No. 5,166,288.
That particular patent discloses that a composition, which contains a high molecular weight polyester (i.e, a number average molecular weight of 15,000 to 30,000) having a glass transition temperature of 5.degree. to 40.degree. C., a melamine resin which contains 60% by weight or more of hexamethoxymethylmelamine monomer, and an amine-blocked derivative of a long-chain alk-ylbenzenesulfonic acid having C.sub.10 -C.sub.20 alkyl group, can be cured to form a coating which is hard, yet flexible, and which resists stains. However, notwithstanding the above, the coating composition disclosed therein has a number of shortcomings associated therewith.
For example, U.S. Pat. No. 5,166,288 reveals that, when the coating is formulated for processability (flexibility) at temperatures below 20.degree. C., the cured coating's stain resistant properties decrease (see, e.g., Example 2, Table 2). This is an expected phenomena since it is known in the coatings industry that ingredients employed to give a coating low temperature flexibility typically reduce the resulting film's stain resistant properties. Accordingly, those in the coatings industry would consider a composition to be an advancement in the art if it can be cured to form a coating which has excellent stain resistant properties and low temperature flexibility (e.g., flexible at temperatures at or below about 10.degree. C.).
Furthermore, it is also known in the coatings industry that the implementation of lower molecular weight binders facilitate the formulation of coatings with a higher solids concentration (i.e., a solids concentration which is greater than about 50 weight percent based upon the total weight of the composition). This higher solids concentration is often a very desirable feature to the end user.
In view of the above, those in the coatings industry would consider a composition to be an advancement in the art if, in addition to having a higher solids concentration, the composition can be cured to form a coating which displays excellent stain resistance and low temperature flexibility properties. It would also be considered as an advancement in the art if, in addition to having the aforementioned properties, the composition can be cured to form a film having a high gloss.